Summary/Abstract Production of high affinity antibody responses depend on productive interaction between antigen specific B cells and follicular helper CD4 T cells (Tfh) in secondary lymphoid tissue. The vast majority of studies performed thus far on the Tfh lineage of CD4 T cells have focused on the key regulatory and genetic events associated with their development. However, little is known about the repertoire selection of Tfh drawn from the polyclonal endogenous pool of CD4 T cells. A currently accepted model for the development of Tfh cells is that they are first primed by encounter with antigen-bearing dendritic cells (DC) and subsequently have their differentiation program solidified by cognate interaction with antigen-specific B cells. We hypothesize that because of the requisite requirement for cognate B cell interaction with CD4 T cells for full Tfh commitment, the peptide specificity of Tfh cells will reflect these differences in epitope display by antigen-specific B cels. We will test this hypothesis through use of two distinct genetic strategies perturb B cell presentation of antigen in vivo. First, we will selectively alter the composition of the antigen processing compartments of B cells through elimination of key endosomally localized thiolreductase within this subset of antigen presenting cells. Second, we will promote immunoglobulin mediated uptake of antigen through a dominant B cell receptor by use of an immunoglobulin transgenic mouse and a set of epitope tagged antigens. We have recently developed experimental strategies to isolate Tfh cells and to analyze the immunodominance hierarchy drawn from the endogenous CD4 repertoire using preparative flow cytometry and peptide-specific cytokine ELISpot assays. We will use these methods in conjunction with the genetic models of B cell presentation of antigen in vivo. In addition to providing direct evidence for the role of B cell epitope display in selecting the repertoire of Tfh cells, and thus clarifyin the final step in Tfh repertoire development, the results of our study will have several important implications for future work. First, they offer a roadmap for enhancing Tfh selection and thus antibody responses. They also provide a model to study the fate of Tfh that fail to be selected by B cells for Tfh repertoire. Finally, these studies may provide insight into CD4 T cell repertoire development when antigen- specific B cells are of limited specificity, as occurs in some autoimmune diseases and in the response to some viral pathogens and vaccines such as HIV, and respiratory viruses such as influenza. We will have developed the model systems and experimental approaches needed to dissect the molecular events in B cell antigen presentation that control follicular helper cell repertoire development.